Field of the Invention
The present invention relates to a control circuit for a variable capacitance element whose capacitance varies with the application of a DC voltage, a resonant circuit including the control circuit, an electronic device, a control method for the variable capacitance element, a control program, and a semiconductor element having the control program. This application is based upon and claims the benefit of priority from the Japanese Patent Application No. 2013-146252 filed in Japan on Jul. 12, 2013, the entire contents of which are incorporated herein by reference.
Description of Related Art
Noncontact communication technology utilizing electromagnetic induction has been increasingly applied to IC cards, including FeliCa™, Mifare™, and NFC (Near Field Communication). The application of this noncontact communication technology is now spreading even to noncontact charging (power feeding) techniques represented by a Qi format, for example.
When noncontact communication is conducted between a noncontact IC card and a reader/writer as well as when noncontact charging in which a relatively large amount of electric power is fed or received is performed, the signal transmission or the power feeding utilizes electromagnetic coupling and a magnetic resonance phenomenon. In such coupling systems, the resonant frequency matching between the resonant circuits on the transmitting and receiving sides is directly linked to a decrease in the number of transmission errors or an improvement in the transmission efficiency. On the other hand, the resonant frequencies of the resonant circuits on the transmitting and receiving sides may vary and fluctuate due to various factors. The capacitances of capacitors and the inductances of coils used in a resonant circuit have initial manufacturing variations, and these variation ranges may be widened because these capacitors and coils each have temperature characteristics according to heat generated during an operation and a surrounding temperature change. Furthermore, the resonant frequency may vary depending on a placement condition of a resonant circuit mounted in a transmitter or a receiver used. A relative positional relationship between the resonant circuits on the transmitting and receiving sides may also influence the transmitting condition.
The resonant frequencies of constituent elements of a resonant circuit are individually tuned before shipping so that the characteristics, including temperature characteristics, of the constituent elements can fall within predetermined ranges. It is, however, extremely difficult to design the circuit in consideration of varying resonant frequencies when the circuit is used. Even when certain levels of initial variations in elements are recognized, excessively reducing these initial variations could not be preferred, because the excessive reduction in the variations may lead to an increase in a material cost and complexity of a manufacturing process.
For the purpose of correcting a variation in a resonant frequency of a resonant circuit and reducing an influence of the variation in the resonant frequency in use, techniques have been studied to automatically tune the resonant frequency on each of the transmitting and receiving sides or automatically tune the resonant frequencies on both the transmitting and receiving sides.
To give some examples, Patent Literature 1 describes a method in which resonant capacitors are mounted in a resonant circuit and a semiconductor switch is used to tune the capacitances of the resonant capacitors such that the resonant circuit has a desired resonant frequency. Patent Literature 2 describes a method in which a variable capacitance capacitor having a ferroelectric thin film is used as a constituent resonant capacitor in a resonant circuit and the capacitance of the resonant capacitor is controlled using an external DC bias such that the resonant circuit has a desired resonant frequency. Moreover, Patent Literature 2 describes a method in which the phase difference between input and output signals of a resonant coil is used to sense the tuned resonant frequency of the resonant circuit and the resonant frequency can thereby be tuned easily without detecting the peak of an analog signal.
Patent Literature 1: JP 2008-160312 A
Patent Literature 2: JP 2012-099968 A